--- a/src/hotspot/share/gc/z/zDirector.cpp	Wed Nov 20 10:37:45 2019 +0100
+++ b/src/hotspot/share/gc/z/zDirector.cpp	Wed Nov 20 10:37:46 2019 +0100
@@ -75,7 +75,7 @@
   // duration, which is needed by the other rules.
   const size_t max_capacity = ZHeap::heap()->soft_max_capacity();
   const size_t used = ZHeap::heap()->used();
-  const double used_threshold_percent = (ZStatCycle::ncycles() + 1) * 0.1;
+  const double used_threshold_percent = (ZStatCycle::nwarmup_cycles() + 1) * 0.1;
   const size_t used_threshold = max_capacity * used_threshold_percent;
 
   log_debug(gc, director)("Rule: Warmup %.0f%%, Used: " SIZE_FORMAT "MB, UsedThreshold: " SIZE_FORMAT "MB",
@@ -85,7 +85,7 @@
 }
 
 bool ZDirector::rule_allocation_rate() const {
-  if (ZStatCycle::is_first()) {
+  if (!ZStatCycle::is_normalized_duration_trustable()) {
     // Rule disabled
     return false;
   }

--- a/src/hotspot/share/gc/z/zDriver.cpp	Wed Nov 20 10:37:45 2019 +0100
+++ b/src/hotspot/share/gc/z/zDriver.cpp	Wed Nov 20 10:37:46 2019 +0100
@@ -250,17 +250,11 @@
   case GCCause::_z_allocation_stall:
   case GCCause::_z_proactive:
   case GCCause::_z_high_usage:
+  case GCCause::_metadata_GC_threshold:
     // Start asynchronous GC
     _gc_cycle_port.send_async(cause);
     break;
 
-  case GCCause::_metadata_GC_threshold:
-    // Start asynchronous GC, but only if the GC is warm
-    if (ZStatCycle::is_warm()) {
-      _gc_cycle_port.send_async(cause);
-    }
-    break;
-
   case GCCause::_gc_locker:
     // Restart VM operation previously blocked by the GC locker
     _gc_locker_port.signal();
@@ -352,13 +346,15 @@
 
 class ZDriverGCScope : public StackObj {
 private:
-  GCIdMark      _gc_id;
-  GCCauseSetter _gc_cause_setter;
-  ZStatTimer    _timer;
+  GCIdMark       _gc_id;
+  GCCause::Cause _gc_cause;
+  GCCauseSetter  _gc_cause_setter;
+  ZStatTimer     _timer;
 
 public:
   ZDriverGCScope(GCCause::Cause cause) :
       _gc_id(),
+      _gc_cause(cause),
       _gc_cause_setter(ZCollectedHeap::heap(), cause),
       _timer(ZPhaseCycle) {
     // Update statistics
@@ -371,7 +367,7 @@
                                 (double)ZHeap::heap()->nconcurrent_no_boost_worker_threads();
 
     // Update statistics
-    ZStatCycle::at_end(boost_factor);
+    ZStatCycle::at_end(_gc_cause, boost_factor);
 
     // Update data used by soft reference policy
     Universe::update_heap_info_at_gc();

--- a/src/hotspot/share/gc/z/zStat.cpp	Wed Nov 20 10:37:45 2019 +0100
+++ b/src/hotspot/share/gc/z/zStat.cpp	Wed Nov 20 10:37:46 2019 +0100
@@ -1024,7 +1024,7 @@
 //
 // Stat cycle
 //
-uint64_t  ZStatCycle::_ncycles = 0;
+uint64_t  ZStatCycle::_nwarmup_cycles = 0;
 Ticks     ZStatCycle::_start_of_last;
 Ticks     ZStatCycle::_end_of_last;
 NumberSeq ZStatCycle::_normalized_duration(0.3 /* alpha */);
@@ -1033,9 +1033,12 @@
   _start_of_last = Ticks::now();
 }
 
-void ZStatCycle::at_end(double boost_factor) {
+void ZStatCycle::at_end(GCCause::Cause cause, double boost_factor) {
   _end_of_last = Ticks::now();
-  _ncycles++;
+
+  if (cause == GCCause::_z_warmup) {
+    _nwarmup_cycles++;
+  }
 
   // Calculate normalized cycle duration. The measured duration is
   // normalized using the boost factor to avoid artificial deflation
@@ -1045,16 +1048,18 @@
   _normalized_duration.add(normalized_duration);
 }
 
-bool ZStatCycle::is_first() {
-  return _ncycles == 0;
+bool ZStatCycle::is_warm() {
+  return _nwarmup_cycles >= 3;
 }
 
-bool ZStatCycle::is_warm() {
-  return _ncycles >= 3;
+uint64_t ZStatCycle::nwarmup_cycles() {
+  return _nwarmup_cycles;
 }
 
-uint64_t ZStatCycle::ncycles() {
-  return _ncycles;
+bool ZStatCycle::is_normalized_duration_trustable() {
+  // The normalized duration is considered trustable if we have
+  // completed at least one warmup cycle
+  return _nwarmup_cycles > 0;
 }
 
 const AbsSeq& ZStatCycle::normalized_duration() {
@@ -1062,8 +1067,8 @@
 }
 
 double ZStatCycle::time_since_last() {
-  if (_ncycles == 0) {
-    // Return time since VM start-up
+  if (_end_of_last.value() == 0) {
+    // No end recorded yet, return time since VM start
     return os::elapsedTime();
   }
 

--- a/src/hotspot/share/gc/z/zStat.hpp	Wed Nov 20 10:37:45 2019 +0100
+++ b/src/hotspot/share/gc/z/zStat.hpp	Wed Nov 20 10:37:46 2019 +0100
@@ -25,6 +25,7 @@
 #define SHARE_GC_Z_ZSTAT_HPP
 
 #include "gc/shared/concurrentGCThread.hpp"
+#include "gc/shared/gcCause.hpp"
 #include "gc/shared/gcTimer.hpp"
 #include "gc/z/zMetronome.hpp"
 #include "logging/logHandle.hpp"
@@ -365,19 +366,21 @@
 //
 class ZStatCycle : public AllStatic {
 private:
-  static uint64_t  _ncycles;
+  static uint64_t  _nwarmup_cycles;
   static Ticks     _start_of_last;
   static Ticks     _end_of_last;
   static NumberSeq _normalized_duration;
 
 public:
   static void at_start();
-  static void at_end(double boost_factor);
+  static void at_end(GCCause::Cause cause, double boost_factor);
 
-  static bool is_first();
   static bool is_warm();
-  static uint64_t ncycles();
+  static uint64_t nwarmup_cycles();
+
+  static bool is_normalized_duration_trustable();
   static const AbsSeq& normalized_duration();
+
   static double time_since_last();
 };